Power supply and image forming device having the same

ABSTRACT

A power supply, which outputs a plurality of voltages in order to improve the cross regulation between output voltages and at the same time reduce the amount of electric power consumed, and an image forming device having the same are disclosed. The power supply includes a power converter, which generates a first output power source and a second output power source in response to an external power supply and a power control signal, respectively; a power output part, which includes output parts to rectify and smooth each of the first and second output power sources; a first output controller, which receives the first output power source feedback from the power output part to generate the power control signal; and a second output controller, which receives the second output power source feedback from the power output part to control to operate the power output part in stable mode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 11/686,422 filed Mar. 15, 2007, now U.S. Pat. No. 7,933,131 andclaims the benefit of Korean Patent Application No. 2006-71768, filedJul. 28, 2006 in the Korean Intellectual Property Office, the disclosureof which is incorporated herein by reference.

BACKGROUND

1. Field

Aspects of the present invention relate to a power supply and an imageforming device having the same, and, more particularly, to a powersupply, which supplies a voltage having a plurality of potential levelsin order to improve cross regulation between outputted voltages and atthe same time reduce electric power consumption, and an image formingdevice having the same.

2. Description of the Related Art

Generally, power supplies using switching methods (hereinafter referredto as “switching power supplies”) have been widely utilized. In theswitching methods, a direct current obtained by rectifying and smoothinga commercial alternating current is switched by a predetermined highfrequency (for example, about 100 kHz) to be converted to a desiredvoltage by a high efficiency transformer. A method for controllingoutput voltages of the switching power supply uses a pulse widthmodulation (PWM) control method to control the duty cycle of a switchingpulse according to the change of output voltage, a frequency controlmethod to control the frequency of the switching pulse, and phasecontrol method to control a phase of the switching pulse.

FIG. 9 is a view to illustrate an example of the switching power supplyusing the PWM control method. The switching power supply 10 in FIG. 9using the PWM control method has a switching circuit 12 including one ormore switches formed on a primary winding of a transformer 11. Byturning the switching circuit 12 on or off, the switching power supply10 converts a direct current input voltage DC_IN, which is applied tothe primary winding of the transformer 11 and is not rectified, to adirect current output voltage DC_OUT. The generated direct currentoutput voltage DC_OUT is rectified by a diode D1 and a capacitor C1inside an output part 13 connected to a secondary winding of thetransformer 11 (Vout) to be outputted. In the device shown, theswitching circuit 12 is turned on or off in accordance with a controlsignal of an output controller 14 to modulate the pulse width of theswitching pulse in response to the output signal from the output part13.

In the switching power supply in FIG. 9, since the rectified endconnected to the secondary winding has a simple structure, a smallnumber of parts are used. Therefore, it is suitable to apply theswitching power supply to a multi-output power supply that outputsvoltages with different potential levels. FIGS. 10 and 11 are views toillustrate examples of conventional multi-output power supplies. In FIG.10, a conventional multi-output power supply 20 according to the firstexample has power supplies 30 and 40. Each of the power supplies 30 and40 has the same constitution as that of the power supply illustrated inFIG. 9. For example, each of the power supplies 30 and 40 has first andsecond input parts 31 and 41 receiving the direct current input voltageDC_IN; first and second power converters 32 and 42 each have thetransformer; first and second output parts 33 and 43 rectifying thedirect current output voltages DC_OUT1 and DC_OUT2 outputted from thepower converters 32 and 42, respectively, to output the direct currentoutput voltages; and first and second output controllers 34 and 44modulating the pulse width in response to the direct current outputvoltages Vout1 and Vout2 outputted from the output parts 33 and 43,respectively, to output the pulse width. When the power supplies 30 and40 are used in an external apparatus (for example the image formingdevice 50 shown, such as a printer,) an On/Off control signal tointerrupt operation of the second power supply 40 is provided to thesecond output controller 44 to prevent an output of the switching pulsefrom the second output controller 44 in a standby mode when a printengine part 52 does not operate.

When the multi-output power supply 20 has the above-described structure,the multi-output power supply 20 has multiple input/output parts 31, 33,41 and 43, the power converters 32 and 42, and the output controllers 34and 44 to multi-output the voltage. Accordingly, a large number of partsare used, increasing the size of the power supply 20 and resulting in anincrease in manufacturing cost.

Referring to FIG. 11, a conventional multi-output power supply 70according to the second example has an input part 71 receiving thedirect current input voltage DC_IN; a power converter 72 having atransformer having one primary winding and two secondary windings; firstand second output parts 73 and 74 rectifying the direct current outputvoltages DC_OUT1 and DC_OUT2 outputted from the power converter 72,respectively, to output the direct current output voltages Vout1 andVout2; and an output controller 75 modulating the pulse width of theswitching pulse in response to the first direct current output voltageVout1 of the second output part 73 to output the pulse width. When themulti-output power supply 70 has the above-described structure, theconstitution of the input part 71 is simpler than the multi-output powersupply 20 illustrated in FIG. 10, and is further simplified by includingone power converter 72 and one output controller 75. Therefore, themanufacturing costs of the power supply 70 can be reduced.

However, in the multi-output power supply 70 according to the secondexample, the pulse width of the switching pulse inputted to the inputpart 71 for controlling the direct current output voltages Vout1 andVout2 is modulated depending on the direct current output voltage Vout1outputted from the first output part 73, so that the voltage stabilityof the direct current output voltage Vout2 of the second output part 74decreases.

In particular, if the pulse width of the switching pulse is varied inorder to compensate for the impedance when the impedance of the load ofthe first output part 73 or the second output part 74 changes, thevoltage stability of the direct current output voltage at the other side(that is, the cross regulation) is reduced because one primary windingis used together with the secondary winding.

SUMMARY OF THE INVENTION

Aspects of the present invention provide a power supply and an imageforming device having the power supply, in which the cross regulation ofa multi-output power supply is improved, and at the same time themulti-output power supply operates in a standby mode to reduce electricpower consumption.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

According to an aspect of the present invention, there is provided animage forming device comprising a power converter, a power output part,a first output controller, and a second output controller. The powerconverter generates a first output power source and a second outputpower source in response to an external power supply and a power controlsignal. The power output part rectifies and smoothes the output firstand second output power sources. The first output controller receivesthe rectified and smoothed first output power source feedback from thepower output part to generate the power control signal. The secondoutput controller receives the rectified and smoothed second outputpower source to control operating modes of the power output part.

According to another aspect of the present invention, the operatingmodes comprise a normal mode, in which the second output power source isoutputted with a level within an allowable error range with respect to areference value; and a stable mode, in which the power output part iscontrolled to output the second output power source with a level withinan allowable error range if the level of the second output power sourceexceeds the allowable error range.

According to another aspect of the present invention, the operatingmodes may also comprise a normal mode, in which the second output powersource is outputted with a level within the allowable error range withrespect to the reference value; and a standby mode, in which the poweroutput part is controlled so that a level of the second output powersource decreases to reduce an amount of electric power consumed.

According to another aspect of the present invention, the power outputpart includes a first output part to rectify and smooth the first outputpower source and a second output part to rectify and smooth the secondoutput power sources. The second output controller connects the firstoutput part and the second output part.

According to another aspect of the present invention, the second outputcontroller comprises a first switching circuit with one end connected tothe first output part, and another end connected to the second outputpart.

According to another aspect of the present invention, the second outputcontroller may further comprise a differential circuit, to receive therectified and smoothed second output power source and to generate a modecontrol signal to operate the power output part in a stable mode of theoperating modes in which the second output power source is outputtedwithin an allowable error range with respect to a reference value of thesecond output power source. The first switching circuit may be activatedin response to the mode control signal.

According to another aspect of the present invention, the second outputcontroller may further comprise a second switching circuit, to activatein response to an external control signal and to generate a mode controlsignal so that the power output part operates in a standby mode of theoperating modes, in which a level of the second output power sourcedecreases to reduce an amount of electric power consumed by the poweroutput part. The first switching circuit may be activated in response tothe mode control signal.

According to another aspect of the present invention, the second outputcontroller further comprises a differential circuit, to receive thefeedback of the second output power source and to generate a first modecontrol signal to operate the power output part in a stable mode of theoperating modes in which the second output power source is outputtedwithin an allowable error range with respect to a reference value; and asecond switching circuit, to activate in response to an external controlsignal and to generate a second mode control signal so that the poweroutput part operates in a standby mode of the operating modes, in whichthe level of the second output power source decreases to reduce anamount of electric power consumed. In this case, the first switchingcircuit may be activated in response to the first or second mode controlsignal.

According to another aspect of the present invention, the differentialcircuit and the second switching circuit may have an o-ring structure toprovide the first and second mode control signals to the first switchingcircuit.

According to another aspect of the present invention, the firstswitching circuit comprises a bypass switch activated in response to thefirst or second mode control signal.

According to another aspect of the present invention, the firstswitching circuit may further comprise a rectifier element connected tothe bypass switch in series to rectify the second output power sourceoutput from the power converter.

According to another aspect of the present invention, the bypass switchoperates in a saturated region when operating in standby mode, andoperates in a linear region between the saturated regions when operatingin stable mode.

According to another aspect of the present invention, the differentialcircuit may generate the first mode control signal using a differencebetween the second output power source and the reference value.

According to another aspect of the present invention, the differentialcircuit may comprise a charge element to prevent excessive current fromflowing into the first switching circuit.

According to another aspect of the present invention, there is providedan image forming device comprising a power supplier, a print controller,and a print engine part. The power supplier generates a first outputpower source and a second output power source in response to an externalpower supply and a power control signal and uses the second output powersource to control operating modes of the power supplier. The printcontroller receives the first output power source to control theprinting and an operating state of the image forming device. The printengine part receives the second output power source and the controlsignal outputted from the print controller to perform printing.

According to another aspect of the present invention, the operatingmodes comprise a normal mode, in which the second output power source isoutputted with a level within an allowable error range with respect to areference value; and a stable mode, in which the second output powersource is outputted with a level within an allowable error range whenthe level of the second output power source exceeds the allowable errorrange.

According to another aspect of the present invention, the operatingmodes may also comprise a normal mode, in which the second output powersource is outputted with a level within an allowable error range withrespect to the reference value; and a standby mode, in which the levelof the second output power source decreases to reduce an amount ofelectric power consumed.

According to another aspect of the present invention, the power suppliercomprises a power converter to generate an unrectified first outputpower source and an unrectified second output power source in responseto an external power supply and a power control signal; a power outputpart to rectify and smooth the output unrectified first and unrectifiedsecond output power sources; a first output controller to receive thefirst output power source from the power output part and generates thepower control signal used at the power converter; and a second outputcontroller to receive the second output power source from the poweroutput part and controls the operating mode of the power output part.

According to another aspect of the present invention, the power outputpart includes a first output part to output the first output powersource, and a second output part to output the second output powersource. The second output controller includes a first switching circuitconnected to the first and second output parts.

According to another aspect of the present invention, the second outputcontroller may further comprise a differential circuit to receive thesecond output power source and to generate a mode control signal tooperate the power output part in a stable mode of the operating modes inwhich the second output power source is outputted within an allowableerror range with respect to a reference value of the second output powersource. The first switching circuit may be activated in response to themode control signal.

According to another aspect of the present invention, the second outputcontroller may further comprise a second switching circuit to activatein response to an external control signal and to generate a mode controlsignal so that the power output part operates in standby mode of theoperating modes, in which a level of the second output power sourcedecreases to reduce an amount of electric power consumed by the poweroutput part. The first switching circuit may be activated in response tothe second mode control signal.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily, appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a block diagram to illustrate an image forming deviceaccording to one embodiment of the present invention;

FIG. 2 is a block diagram to illustrate one embodiment of a powersupplier illustrated in FIG. 1;

FIG. 3 is a block diagram to illustrate in detail one embodiment of asecond output controller illustrated in FIG. 2;

FIG. 4 is a circuit diagram to illustrate one embodiment of a powersupplier illustrated in FIG. 2;

FIG. 5 is a graph to explain the operation of a first switching circuitillustrated in FIG. 4;

FIGS. 6 and 7 are waveform diagrams to explain the operation of thesecond output controller illustrated in FIG. 5 in a stable mode;

FIG. 8 is a waveform diagram to explain the operation of the secondoutput controller illustrated in FIG. 5 in a standby mode;

FIG. 9 is a view to illustrate an example of a switching power supplyusing a PWM control method; and

FIGS. 10 and 11 are views to illustrate examples of conventionalmulti-output power supplies.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

FIG. 1 is a block diagram illustrating an image forming device 1000according to one embodiment of the present invention. Other aspects ofthe present invention may be used in any electric device, such aspersonal computers. In FIG. 1, an image forming device 1000 according toone embodiment of the present invention comprises an AC/DC converter101, a power supplier 100, a print controller 200 and a print enginepart 300. Specifically, the power supplier 100 has a switching modepower supply (hereinafter, referred to as “SMPS”). The SMPS receives aDC power source converted by the AC/DC converter 101, which converts anAC power source applied from the outside to a DC power source used bythe power supplier 100. The converted DC power source falls below apredetermined level to be provided to each part inside the image formingdevice 1000. The AC/DC converter 101 can be any mechanism which performsa conversion of the AC power source to the DC power source, such as abridge rectifier circuit or the like.

The power supplier 100 receives the DC input power source DC_IN andgenerates a plurality of DC output power sources Vout1 and Vout2 to beoutputted. In other words, in order to generate and provide a ratedvoltage used in each part of the image forming device 1000, the powersupplier 100 according to one embodiment of the present inventiongenerates a plurality of voltages to be outputted. For example, theprint controller 200 may include a microcontroller and circuit elementsconnected to the microcontroller, and the power supplier 100 maygenerate a first output voltage Vout1 within the allowable error rangebased on 5V to output the voltage when a rated voltage of 5V is used inthe microcontroller and the circuit elements. Additionally, the printengine part 300 may include an operating motor to perform printing, atransfer roller connected to the motor, or the like. When a ratedvoltage of 24V is used to operate the operating motor and to heat thetransfer roller, the power supplier 100 generates a second outputvoltage Vout2 within the allowable error range based on 24V to outputthe voltage.

In particular, the power supplier 100 according to an embodiment of thepresent invention may comprise one transformer including one primarywinding and a plurality of secondary windings facing the primarywinding. Additionally, the power supplier 100 operates in stable mode toimprove the cross regulation between the first or second output voltagesVout1 and Vout2. When either the first or second output voltage Vout1 orVout2 is not used (for example when it does not matter that printing isnot performed or to stop operation of the print engine part 300), thepower supplier 100 operates in standby mode.

The print controller 200 operates by the first output voltage Vout1outputted from the power supplier 100 to generate a drive control signalCS_drv controlling each constituent element of the print engine part300, and to control the whole operation of the image forming device1000. In other words, the print controller 200 controls the wholeoperation of the print engine part 300, which loads printing paper,transfers paper, forms images on printing paper based on the print data,fixes the printed images, discharges the printing results, and/orperforms other functions, depending on the nature of the image formingapparatus. The print controller 200 may also control the operating stateof the image forming device 1000 by determining printing errors such aspaper jams.

Additionally, the print controller 200 generates a standby mode controlsignal CS_stb to be outputted in order to operate the power supplier 100in standby mode. The standby mode control signal CS_stb may be appliedby a user, or be automatically outputted if it the print controller 200,or other component, determines that processing of the print data iscompleted. For example, when the image forming device 1000 is a laserprinter, the print engine part 300 comprises a fixer comprising aphotosensitive drum (OPC drum), a developing unit, a fixing unit or thelike, and a laser scanning unit (LSU) irradiating a laser beam on thephotosensitive drum. Each part of the print engine part 300 is operatedby the second output voltage Vout2 outputted from the power supplier 100and the drive control signal CS_drv outputted from the print controller200 to form predetermined images on a print medium based on the printdata.

FIG. 2 is a block diagram illustrating an embodiment of the powersupplier 100 shown in FIG. 1. In FIG. 2, the power supplier 100comprises a power converter 120, a power output part 140, a first outputcontroller 160, and a second output controller 180. Specifically, thepower converter 120 decreases the DC input voltage DC_IN below apredetermined level, and outputs output voltages Vout1′ and Vout2′respectively having a plurality of potential levels.

The power converter 120 comprises a transformer, including one primarywinding and a plurality of secondary windings, all of which are usedtogether with the primary winding in order to provide the plurality ofoutput voltages, and at the same time to reduce the number of theelements used in the power supplier 100 and to reduce the size of thepower supplier 100.

The power converter 120 further comprises a switching circuit, whichreceives a switching pulse signal and controls the voltage applied tothe primary winding according to the pulse width of the switching pulsesignal to form an induced voltage variably.

The power output part 140 comprises output parts 142 and 144, whichreceive the output voltages Vout1′ and Vout2′, which are output from thepower converter 120 and which are not rectified and smoothed. The outputparts 142, 144 output the output voltages Vout1 and Vout2, respectively,which are rectified and smoothed.

When the first output controller 160 receives first output voltage Vout1feedback from the first output part 142 and the first output voltageVout1 is changed, the first output controller 160 outputs a firstcontrol signal CS1 controlling the operation of the power converter 120.At this time, the first control signal CS1 is the switching pulsesignal, and the pulse width of the first control signal CS1 is modulatedbased on the electric potential change of the first output voltageVout1. The first control signal CS1 having the modulated pulse width isprovided to the power converter 120 to control the potential of thefirst and second output voltages Vout1′ and Vout2′, which are notrectified or smoothed.

When the second output controller 180 receives second output voltageVout2 feedback from the second output part 144, the second outputvoltage Vout2 is compared to a reference voltage Vref. If the secondoutput voltage Vout2 exceeds the maximum value of an allowable errorrange, the power supplier 140 operates in a stable mode to improve thecross regulation of the first and second output voltages Vout1 andVout2.

Additionally, the second output controller 180 receives the standby modecontrol signal CS_stb outputted from the print controller 200illustrated in FIG. 1, and an operating voltage Vcc of a predeterminedlevel to operate the power output part 140 in standby mode.

FIG. 3 is a block diagram illustrating in detail the second outputcontroller 180 illustrated in FIG. 2. The second output controller 180according to one embodiment of the present invention comprises a firstswitching circuit 182, a differential circuit 184, and a secondswitching circuit 186. Specifically, the differential circuit 184compares the second output voltage Vout2 outputted from the secondoutput part 144 to the reference voltage Vref. If the second outputvoltage Vout2 exceeds the maximum value of the allowable error rangebased on the reference voltage Vref, the differential circuit 184outputs a stable mode control signal CS_sta to operate the powersupplier 140 in stable mode.

The second switching circuit 186 receives the power voltage Vcc and thestandby mode control signal CS_stb outputted from the print controller200 to selectively output the power voltage Vcc or a ground voltage GND.

The first switching circuit 182 operates in the stable mode in responseto the stable mode control signal CS_sta outputted from the differentialcircuit 184, or operates in standby mode in response to the standby modecontrol signal CS_stb outputted from the second switching circuit 186.

Additionally, when the first switching circuit 182 does not operate inthe stable mode, the differential circuit 184 outputs a signal of thepotential level to deactivate the first switching circuit 182. When thefirst switching circuit 182 does not operate in the standby mode, thesecond switching circuit 186 also outputs a signal of the potentiallevel to deactivate the first switching circuit 182. Accordingly, thefirst switching circuit 182 is deactivated, and the power supplier 140operates in the normal mode.

In normal mode, the power supplier 140 normally outputs the first andsecond output voltages Vout1 and Vout2, and operates in a state in whichthe second output voltage Vout2 outputted from the second output part144 has a potential level within a range not exceeding the allowableerror range.

The constitution and operation of the second output controller 180 isdescribed in more detail as follows, with reference to FIGS. 4 through8. FIG. 4 is a circuit diagram illustrating an embodiment of the powersupplier 100 illustrated in FIG. 2. FIG. 5 is a graph to explain theoperation of the first switching circuit 182 illustrated in FIG. 4. Inaddition, FIGS. 6 and 7 are waveform diagrams to explain the operationof the second output controller 180 illustrated in FIG. 5 in stablemode, and FIG. 8 is a waveform diagram to explain the operation of thesecond output controller 180 illustrated in FIG. 5 in standby mode.

Referring to FIGS. 2 and 4, the power supplier 100 according to anembodiment of the present invention comprises the power converter 120,the power output part 140, the first output controller 160, and thesecond output controller 180. The power converter 120 comprises a firstpower converter 122, a second power converter 124, and a first switchingelement Q1. The power output part 140 comprises the first and secondoutput parts 142 and 144. The first output controller 160 is connectedto the first output part 142 and the first switching element Q1. Thesecond output controller 180 is connected to the first and second outputparts 142 and 144, and comprises the first switching circuit 182, thedifferential circuit 184, and the second switching circuit 186.

Specifically, the power converter 120 comprises the primary winding andthe first switching element Q1 connected to the primary winding. Thefirst power converter 122 comprises a first secondary winding facing theprimary winding, and the second power converter 124 comprises a secondsecondary winding facing the primary winding. The first control signalCS1 outputted from the first output controller 160 is applied to thegate terminal of the first switching element Q1 to perform switching.

The first output part 142 comprises a first diode D1 and a firstcapacitor C1. The second output part 144 comprises a second diode D2 anda second capacitor C2. The diodes D1 and D2 and the capacitors C1 and C2rectify and smooth the direct current output voltages Vout1′, Vout2′induced from the first and second power converters 122 and 124,respectively.

The first output controller 160 is connected to a first node N1 of thefirst output part 142 to receive the first output voltage Vout1 and tooutput the first control signal CS1 as a switching pulse signal havingthe pulse width modulated accordingly. The switching pulse signalcontrols the direct current input voltage DC_IN to be applied to theprimary winding.

The second output controller 180 comprises the differential circuit 184to operate in the stable mode and the second switching circuit 186 tooperate in the standby mode. In other embodiments, the second outputcontroller 180 may comprise only the differential circuit 184, only thesecond switching circuit 186, or both the differential circuit 184 andthe second switching circuit 186.

The differential circuit 184 is connected to a third node N3 of thesecond output part 144 to receive the second output voltage Vout2.Additionally, the differential circuit 184 comprises an error detectioncircuit OP-AMP, which compares the reference voltage Vref to thereceived second output voltage Vout2. Integrated circuits R3 and C3integrate the detected error voltage. Allotter circuits R4 and R5 dividethe result of integration and output the second control signal CS2 tooperate the first switching circuit 182 with phase 1. A charge elementC4 performs a soft start function to prevent the first switching circuit182 from being damaged due to excessive current flowing through thefirst switching circuit 182 as a result of the first switching circuit182 being turned on or off suddenly.

The second switching circuit 186 comprises a third switching element Q3,and activates the third switching element Q3 with phase 2 to output thepower voltage Vcc to the second control signal CS2 when operating in thestandby mode in response to the standby mode control signal CS_stbapplied from the outside. Additionally, when the power supplier 140operates in the normal mode or the stable mode, the second switchingcircuit 186 prevents the output of the power voltage Vcc to the secondcontrol signal CS2 by activating the third switching element Q3.

The first and third switching elements Q1 and Q3 can variously comprisean npn type transistor, a pnp type transistor, or the like.

The first switching circuit 182 is connected to a second node N2 of thefirst output part 142 and a fourth node N4 of the second output part144. The first switching circuit 182 comprises a third diode D3rectifying the first output voltage Vout1′, which is divided in thefourth node N4 and is not rectified, and a second switching element Q2activated with phase 1 or phase 2 in response to the second controlsignal CS2 outputted from the differential circuit 184 or the secondswitching circuit 186. The second control signal CS2 outputted from thedifferential circuit 184 or the second switching circuit 186 may have ano-ring structure to be commonly inputted into the gate terminal of thesecond switching element Q2. Other aspects may use other structures.

The second switching element Q2 may comprise a bypass switch, but is notrestricted thereto.

The second switching element Q2 operating with phase 1 and phase 2 isdescribed below with reference to FIG. 5. The bypass switch Q2 operateswith phase 1 operating in an active region indicated by oblique lines,and phase 2 operating in a saturated region, which is at one end of thestraight line.

When operating with phase 1, a predetermined current passes through adrain-source terminal of the bypass switch Q2 if a potential of apredetermined level is applied to the bypass switch Q2. When operatingwith phase 2, the bypass switch Q2 is turned on or off so that the firstswitching circuit 182 allows the second and fourth nodes N2 and N4 to beshort or open. The bypass switch Q2 may comprise an NMOS transistor or aPMOS transistor, but is not restricted thereto.

A method for operating the power output part 140 in stable mode isdescribed below with reference to FIGS. 4, 6 and 7 as follows. Referringto FIG. 6, the normal mode, in which the second output voltage Vout2outputted from the second output part 144 has a potential level within arange not exceeding the allowable error range, is indicated by a timepoint t1. Until reaching the time point t1, the difference between thevoltages outputted from the differential circuit 184 has a negativevalue to output a voltage of 0V. Accordingly, the differential circuit184 does not output the second control signal CS2 and a current Ipcaused by the direct current input voltage DC_IN is applied to theprimary winding according to the switching operation of the firstswitching element Q1. Therefore, the induced voltages are formed in eachsecondary windings of the first and second power converters 124 and 126in response to the winding ratio of the primary winding to the secondarywinding.

The induced voltages formed in each of the first and second powerconverters 124 and 126 is rectified and smoothed by the diode D1 and thecapacitor C1 of the first output part 142 and the diode D2 and thecapacitor C2 of the second output part 144, respectively, to beoutputted as a direct current output voltage Vout1 and Vout2,respectively.

Subsequently, when the impedance of the load of the second output part144 changes between the time points t1 to t2, the second output voltageVout2 outputted from the second output part 144 begins to increase. Attime point t2, the second output voltage Vout2 reaches the maximum valueVref_max of the allowable error range, the second control signal CS2having a uniform time constant is outputted from the error detectioncircuit OP-AMP by a time constant of the integrated circuits R3 and C3.In this situation, since the second control signal CS2 changes accordingto the second output voltage Vout2, the second switching element Q2operates in a linear region of the straight line illustrated in FIG. 5so that the second switching element Q2 is equal to the referencevoltage Vref.

When the error detection circuit OP-AMP operates, the impedance betweendrain and source of the second switching element Q2 of the firstswitching circuit 182 decreases to increase a current IQ2 flowing in thesecond switching element Q2 and moving from the fourth node N4 to thesecond node N2. Accordingly, a current ID2 flowing in the second diodeD2 of the second output part 144 decreases, and as a result, the secondoutput voltage Vout2 is outputted within a range not exceeding themaximum value Vref_max of the allowable error range for the referencevoltage Vref at a time point t3. Therefore, output of excessive voltagesin the second output part 144 is prevented.

Referring to FIG. 7, the second control signal CS2 is applied to thesecond switching element Q2 to operate the power output part 140 in thestable mode. A current ID1 is induced from the current Ip applied to theprimary winding of the power converter 120 by the direct current inputvoltage DC_IN, and then applied to the first output part 142. When thefirst switching circuit 182 operates with phase 1, the current ID1decreases by the amount of the current IQ2 flowing in the secondswitching element Q2, and is then applied. Additionally, when the firstswitching circuit 182 operates, the current ID2 flowing in the secondoutput part 144 also decreases by the amount of the current IQ2 flowingin the second switching element Q2, and is applied.

A method for operating the power output part 140 in standby mode isdescribed below with reference to FIGS. 4 and 8. Operating untilreaching time point t1, that is, the normal mode, in which the secondoutput voltage Vout2 outputted from the second output part 144 has apotential level within a range not exceeding the allowable error range,is performed in the same manner as in the description of FIG. 6. Whenthe standby mode control signal CS_stb is applied from the outside attime point t1, the third switching element Q3 of the second switchingcircuit 186 turns off. After time point t1, that is when the poweroutput part 140 operates in standby mode, the third switching element Q3is turned on or off. In this case, FIG. 8 illustrates that the standbymode control signal CS_stb changes from a ‘high’ logical level to a‘low’ logical level to turn off the third switching element Q3, but thestandby mode control signal CS_stb may change in another fashionaccording to the third switching element Q3 and the second switchingelement Q2.

When the third switching element Q3 is turned off, a fifth node N5 isformed with the potential level of the power voltage Vcc, andaccordingly the second control signal CS2 is outputted with thepotential level of the power voltage Vcc to turn on the second switchingelement Q2 of the first switching circuit 182. At this time, the secondswitching element Q2 operates in the saturated region illustrated inFIG. 5.

When the second switching element Q2 operates in the saturated region,the first switching circuit 182 is activated to form the fourth andsecond nodes N4 and N2 having the same potential level. Accordingly, thecurrent IQ2 applied to the second switching element Q2 increases, andthus, the current ID1 applied to the load through the first diode D1 inthe first power converter 122 and the current ID2 applied to the loadthrough the second diode D2 in the second power converter 124 decrease.Accordingly, the second output voltage Vout2 outputted from the secondoutput part 144 has substantially the same potential level as that ofthe first output voltage Vout1 outputted from the first output part 142,so that the leakage current occurring in the second output part 144 canbe reduced. Therefore, the amount of electric power consumed in thesecond output part 144 is reduced.

As described above, according to aspects of the present invention, it ispossible to improve the cross regulation between a output voltageoutputted from an output part, which does not independently include theoutput controller to control the pulse width, and another output voltageoutputted from another output part in the multi-output power supply.

Additionally, in aspects of the invention, the multi-output power supplyoperates in standby mode, and the output voltage, which is not used,falls below the predetermined level to output the voltage, so that theamount of electric power consumed in the power supply can be reduced.

Furthermore, in aspects of the invention, the current capacity of theswitch is reduced using the bypass switch to remove the constitution ofa switch heat insulation means, and the manufacturing cost of the powersupply can be reduced accordingly.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

What is claimed is:
 1. An image forming device receiving print data toperform printing comprising: a power supplier to generate a first outputpower source and a second output power source in response to an externalpower supply and a power control signal and to use the second outputpower source to control operating modes of the power supplier; a printcontroller to receive the first output power source to control theprinting and an operating state of the image forming device; and a printengine part to receive the second output power source and the controlsignal outputted from the print controller to perform printing, whereinthe power supplier comprises a power converter to generate anunrectified first output power source and an unrectified second outputpower source in response to the external power supply and the powercontrol signal; a power output part to rectify and smooth the outputunrectified first and unrectified second output power sources to producethe first and second output power sources; a first output controller toreceive the first output power source from the power output part andgenerates the power control signal used at the power converter; and asecond output controller to receive the second output power source fromthe power output part and to control the operating modes of the poweroutput part, wherein the power output part comprises a first output partto rectify and smooth the first output power source, and a second outputpart to rectify and smooth the second output power sources, the secondoutput controller connects the first output part and the second outputpart, and the second output controller comprises a first switchingcircuit with one end connected to the first output part and another endis connected to the second output part.
 2. The image forming device asclaimed in claim 1, wherein the operating modes comprise: a normal mode,in which the second output power source is outputted with a level withinan allowable error range with respect to a reference value; and a stablemode, in which the second output power source is outputted with thelevel within an allowable error range when a level of the second outputpower source exceeds the allowable error range.
 3. The image formingdevice as claimed in claim 1, wherein the operating modes comprise: anormal mode, in which the second output power source is output with alevel within an allowable error range with respect to a reference value;and a standby mode, in which a level of the second output power sourcedecreases to reduce an amount of electric power consumed.
 4. The imageforming device as claimed in claim 1, wherein: the second outputcontroller further comprises a differential circuit to receive thesecond output power source and to generate a mode control signal tooperate the power output part in a stable mode of the operating modes inwhich the second output power source is outputted within an allowableerror range with respect to a reference value of the second output powersource; and the first switching circuit is activated in response to themode control signal.
 5. The image forming device as claimed in claim 1wherein: the second output controller further comprises a secondswitching circuit to activate in response to an external control signaland to generate a mode control signal so that the power output partoperates in a standby mode of the operating modes, in which a level ofthe second output power source decreases to reduce an amount of electricpower consumed by the power output part; and the first switching circuitis activated in response to the second mode control signal.
 6. The imageforming device as claimed in claim 1, wherein the second outputcontroller further comprises: a differential circuit to receive therectified and smoothed second output power source and to generate afirst mode control signal to operate the power output part in a stablemode of the operating modes in which the second output power source isoutputted within an allowable error range with respect to a referencevalue; and a second switching circuit to activate in response to anexternal control signal and to generate a second mode control signal sothat the power output part operates in a standby mode of the operatingmodes, in which a level of the second output power source decreases toreduce an amount of electric power consumed, wherein the first switchingcircuit is activated in response to the first or second mode controlsignal.
 7. A device to perform a plurality of functions comprising: apower supplier to generate a first output power source and a secondoutput power source in response to an external power supply and a powercontrol signal, to rectify and smooth a first output power source and asecond output power source, the first output power source and the secondoutput power source being rectified and smoothed by first and secondoutput parts, respectively; a controller to receive the first outputpower source to control the functions and the operating state of thedevice; and a component to receive the second output power source andthe control signal outputted from the controller to perform a functionof the device, wherein the power supplier controlling operating modes ofthe power supply based on the rectified and smoothed second output powersource by using a second output controller to connect the first outputpart and the second output part, the second output controller includinga first switching circuit with one end connected to the first outputpart and another end connected to the second output part.
 8. An imageforming device comprising: a power converter to generate a first outputpower source and a second output power source in response to an externalpower source and a power control signal; a first output part to rectifyand smooth the first output power source; a second output part torectify and smooth the second output power source; a first outputcontroller to receive the rectified and smoothed first output powersource from the first output part to generate the power control signalused at the power converter; a second output controller to connect thefirst output part and the second output part, and to receive therectified and smoothed second output power source from the power outputpart to control operating modes of the first output part and the secondoutput part; a print controller to receive the first output power sourceto control the printing and an operating state of the image formingdevice; and a print engine part to receive the second output powersource and the control signal outputted from the print controller toperform printing, wherein the second output controller comprises adifferential circuit to receive the rectified and smoothed second outputpower source and to generate a first mode control signal to operate thefirst output part and the second output part in a stable mode in whichthe second output power source is output within an allowable error rangewith respect to a predetermined reference value of the second outputpower source, and a first switching circuit comprising a bypass switchactivated in response to the first mode control signal.
 9. The imageforming device as claimed in claim 8, wherein the operating modescomprise: a normal mode, in which the second output power source isoutputted with a level within an allowable error range with respect to areference value; and a stable mode, in which the power output part iscontrolled to output the second output power source with a level withinan allowable error range when a level of the second output power sourceexceeds the allowable error range.
 10. The image forming device asclaimed in claim 8, wherein the operating modes comprise: a normal mode,in which the second output power source is output with a level withinthe allowable error range with respect to the reference value; and astandby mode, in which the power output part is controlled so that alevel of the second output power source decreases to reduce an amount ofelectric power consumed.
 11. The image forming device as claimed inclaim 8, wherein the second output controller comprises a firstswitching circuit with one end connected to the first output part andanother end is connected to the second output part.
 12. The imageforming device as claimed in claim 11, wherein the first switchingcircuit is activated in response to the first mode control signal. 13.The image forming device as claimed in claim 8, wherein the firstswitching circuit further comprises a rectifier element connected to thebypass switch in series to rectify the second output power source outputfrom the power converter.
 14. The image forming device as claimed inclaim 13, wherein the bypass switch operates in a saturated region whenoperating in standby mode, and operates in a linear region between thesaturated regions when operating in stable mode.
 15. The image formingdevice as claimed in claim 12, wherein the differential circuitgenerates the mode control signal using a difference between the secondoutput power source and the reference value.
 16. The image formingdevice as claimed in claim 15, wherein the differential circuitcomprises a charge element to prevent excessive current from flowinginto the first switching circuit.